FMX stereophonic receiver

ABSTRACT

An FMX stereophonic receiver receives an FMX stereophonic broadcast signal which includes a stereo sum signal, an uncompressed stereo difference signal, and a compressed stereo difference signal which is formed by modulating the uncompressed stereo difference signal by a quadrature modulation and being compressed. The FMX stereophonic receiver includes an FM detector for producing an FM detection signal including a stereo pilot signal, a PLL circuit for producing a signal which is in a synchronized relationship with the stereo pilot signal included in the FM detection signal, a synchronous detection circuit for receiving the FM detection signal and for producing the uncompressed stereo difference signal in accordance with the signal produced from the PLL circuit, a quadrature detection circuit for receiving the FM detection signal and for producing the compressed stereo difference signal in accordance with the signal produced from the PLL circuit, and a phase shifting means for shifting the phase of at least one of the FM detection signal applied to the PLL circuit and the FM detection signal applied to the detection circuits so as to correct the phase difference between the synchronous detection signal and the signal to be detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an FMX stereophonic receiver and, moreparticularly, to an FMX stereophonic receiver able to preventdeterioration of stereophonic channel separation caused by phaseshifting.

2. Description of the Prior Art

FMX stereophonic broadcasting has been proposed as one means ofenlarging the service area and improving the signal-to-noise ratiocharacteristics of FM stereo broadcasts. The transmission signal of theaforementioned FMX stereo broadcast includes a compressed stereodifference signal (L-R)' broadcast simultaneously with the transmissionsignal of conventional FM stereo broadcasting, for example, a stereo sumsignal (L+R) and stereo difference signal (L-R). The transmission signalcan be expressed as:

    f(t)=(L+R)+Psin(w/2)t+(L-R)sinwt+(L-R)'coswt               (1)

where L+R is a stereo sum signal, L-R is a stereo difference signal, Pis a stereo pilot signal, and w is the subcarrier angular frequency. Asshown by aforementioned Equation (1), compressed stereo differencesignal (L-R)' is quadrature modulated from uncompressed stereodifference signal (L-R), resulting in an FMX stereo broadcasttransmission signal spectrum shown in FIG. 1.

Furthermore, the relationship between the uncompressed stereo differencesignal (L-R) and the compressed stereo difference signal (L-R)' is asshown in FIG. 2 which expresses the compression characteristics. In FIG.2, when the input signal level is low, the aforementioned signal (L-R)'is 20 dB greater than the uncompressed stereo difference signal (L-R)and, at the same time, input/output characteristics become linear, andalso the compression ratio becomes 1:1. When the level of the inputsignal is medium (approximately -30 dB), the compression ratio becomes∞:1, and input/output characteristics are flat over a range ofapproximately 10 dB. When the input signal level becomes high, theaforementioned signal (L-R)' rapidly attenuates. Therefore, compressedstereo difference signal (L-R)' is as shown by solid line B in FIG. 2with respect to stereo difference signal (L-R) (solid line A), and thesum signal of the aforementioned signal (L-R) and the aforementionedsignal (L-R)' is as shown by dotted line C in FIG. 2.

As discussed above, the transmission signal for FMX stereophonicbroadcasting is received by a receiver as shown in FIG. 3. In FIG. 3,the FMX stereophonic broadcast transmission signal received by antenna 1is received by a receiving circuit 2 of the same construction as aconventional FM stereophonic receiver in which stereo sum signal (L+R)(hereafter referred to as M), stereo difference signal (L-R) (hereafterreferred to as S), and compressed stereo difference signal (L-R)'(hereafter referred to as S') are each demodulated. When the receivedsignal is detected by the FM detection circuit included in the receivingcircuit, stereo sum signal M is demodulated. When the stereo compositesignal is detected by the synchronous detection using the 38-kHzsubcarrier signal obtained from the PLL in the receiving circuit,uncompressed stereo difference signal S is demodulated. And when thestereo composite signal is detected by the quadrature detection,compressed stereo difference signal S' is demodulated.

Uncompressed and compressed stereo difference signals S and S' obtainedfrom receiving circuit 2 are added by adder 3, and the result is appliedto VCA (voltage control amplifier) 4 operating as an attenuator. Whenstereo difference signal S and output signal (S+S') of VCA 4 are greaterthan a specified level (a knee-point level), first and second leveldetection circuits 5 and 6, each having a threshold level, operate insuch a manner that the level of stereo difference signal S and the levelof aforementioned output signal (S+S') of VCA 4 are respectivelydetected by first and second level detection circuits 5 and 6, and arecompared by comparator circuit 7. Next, a signal according to the leveldifference obtained from aforementioned comparator circuit 7 isrectified and smoothed by rectifying circuit 8, and the rectified signalis applied to VCA 4 as a control signal. The output signal (S+S') ofaforementioned VCA 4 is controlled by this control signal to be equal tothe level of stereo difference signal S. However, when aforementionedstereo difference signal S and output signal (S+S') of VCA 4 are belowthe knee-point level, first and second level detection circuits 5 and 6do not operate, and attenuation at VCA 4 is fixed at approximately 20dB.

Although stereo sum signal M obtained from receiving circuit 2 isapplied directly to matrix circuit 9, stereo difference signal S oroutput signal (S+S') of VCA 4 are selected by switch 10, and applied tomatrix circuit 9. Although not given in the above description, a 10-HzID signal is included in the FMX stereophonic broadcast transmissionsignal, and FMX stereophonic broadcasts are differentiated fromconventional FM stereophonic broadcasts by the aforementioned ID signal.In addition, because a detection circuit which detects theaforementioned ID signal is built in to receiving circuit 2, whether thebroadcast is FMX stereo or not can be determined with the output signalof the aforementioned detection circuit. Switch 10 is controlled by theaforementioned ID signal. When the ID signal is present, switch 10 isswitched to a position as shown in FIG. 3. Accordingly, stereo sumsignal M and output signal (S+S') from level controlled VCA 4 arematrixed, and left and right stereo signals L and R are generated atleft and right output terminals 11 and 12. Furthermore, when the IDsignal is not present, switch 10 is switched to a position opposite tothat shown in FIG. 3, and stereo sum signal M and stereo differencesignal S are matrixed in matrix circuit 9.

As described above, because FMX stereophonic broadcast system usescompressed and expanded stereo difference signal S, it is possible toachieve significant improvements in the S/N ratio, and the service areacan be enlarged comparably equal to that of the conventional monaural FMbroadcast system.

It is to be noted that the FMX stereophonic broadcast transmissionsignal can be accurately received by a conventional FM stereophonicreceiver. In this case, compressed stereo difference signal S' isquadrature modulated with respect to stereo difference signal S, andreception is not adversely affected.

Details concerning FMX stereophonic broadcasting are disclosed, forexample, in an article "Improving the Signal-to-Noise Ratio and Coverageof FM Stereophonic Broadcasts" by Emil L. Torick and Thomas B. Keller in"JOURNAL OF THE RADIO ENGINEERING SOCIETY", volume 33, number 12, issuedDecember 1985.

Because FMX stereophonic broadcasting is currently in the experimentalstage, and there is no current broadcasting available, FMX stereophonicreceivers are, of course, not commercially available. However, when areceiver in FIG. 3 was actually designed and experimented with a testsignal to measure the characteristics, channel separation during FMXstereophonic broadcast reception was found to deteriorate. Specifically,uncompressed stereo difference signal S and compressed stereo differencesignal S' are synchronously detected and quadrature detected,respectively, using a signal obtained from a PLL circuit locked to a19-kHz pilot signal included in an FM detection output signal. However,if the phase relationship between the phases of the detection signal andthe signal to be detected undesirably deviates due to the phasecharacteristics of a frequency divider provided in a PLL circuit, or dueto an offset of the phase comparator provided in the PLL circuit, normaldetection output cannot be obtained. Accordingly, when output signal(S+S') of VCA 4 and stereo sum signal M are matrixed, separation can notbe accomplished in a desired form. A 38-kHz detection signal obtainedfrom the PLL circuit may generally cause a phase shift of about 10degrees with respect to the 38-kHz subcarrier, but in FMX broadcastsystem, such a phase shift will result in a great deterioration of thechannel separation, because in FMX broadcast system, a phase shift of 2degrees will result in channel separation deterioration by over 10 dB.

Furthermore, stereo sum signal M, uncompressed stereo difference signalS, and compressed stereo difference signal S' obtained from thereceiving circuit each contain a high harmonic frequency component. In astandard FM stereophonic receiver, there are no particular problems evenwhen the signals are matrixed containing the high frequency component.Nevertheless, in the FMX stereophonic receiver, because the level ofoutput signal (S+S') of VCA 4 is controlled according to the level ofuncompressed stereo difference signal S, if the high frequency componentis applied to the first level detection circuit 5, which detects theuncompressed stereo difference signal S level, the level of the outputsignal (S+S') of VCA 4 changes, and stereo separation during FMX stereobroadcast reception deteriorates.

Moreover, in the circuit of FIG. 3, there is the additional problem ofthe circuit becoming complex because two level detection circuits arerequired, and it is therefore necessary to match the characteristics ofboth level detection circuits.

SUMMARY OF THE INVENTION

The present invention has been developed with a view to substantiallysolving the above described problems and has for its essential object toprovide an improved FMX stereophonic receiver.

It is also an essential object of the present invention to provide anFMX stereophonic receiver of the above described type which can preventthe deterioration of the received signal.

It is also an essential object of the present invention to provide anFMX stereophonic receiver of the above described type which can beeasily manufactured.

In accomplishing these and other objects, an FMX stereophonic receiveraccording to the present invention comprises an FM detector forproducing an FM detection signal including a stereo pilot signal; a PLLcircuit for producing a signal which is in a synchronized relationshipwith the stereo pilot signal included in the FM detection signal; asynchronous detection circuit for receiving the FM detection signal andfor producing the uncompressed stereo difference signal in accordancewith the signal produced from the PLL circuit; a quadrature detectioncircuit for receiving the FM detection signal and for producing thecompressed stereo difference signal in accordance with the signalproduced from the PLL circuit; and a phase shifting means for shiftingthe phase of at least one of the FM detection signal applied to the PLLcircuit and the FM detection signal applied to the detection circuits soas to correct the phase difference between the synchronous detectionsignal and the signal to be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings throughout which like parts are designated by like referencenumerals, and in which:

FIG. 1 is a graph showing an FMX stereo broadcast transmission signalspectrum;

FIG. 2 is a graph showing input/output characteristics of stereodifference signals;

FIG. 3 is a circuit diagram of an FMX stereophonic receiver according toprior art;

FIG. 4 is a circuit diagram of an FMX stereophonic receiver according toa first embodiment of the present invention;

FIG. 5 is a circuit diagram of a phase delay circuit used in the circuitof FIG. 4;

FIG. 6 is a circuit diagram of a phase advance circuit used in thecircuit of FIG. 4;

FIG. 7 is a circuit diagram of an FMX stereophonic receiver accordingthe a second embodiment of the present invention;

FIG. 8 is a circuit diagram of an FMX stereophonic receiver accordingthe a third embodiment of the present invention and;

FIG. 9 is a circuit diagram similar to FIG. 3 illustrating an alternateconstruction for achieving the same result.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 4 shows a first embodiment of the present invention isshown. In the first embodiment, an improvement is made in receivingcircuit 2 shown in FIG. 3. In FIG. 4, a reference number 13 designatesan FM detection circuit for the FM detection of an IF signal applied toan input terminal 14, and a reference number 15 designates a PLL circuitwhich generates an output signal synchronized to a 19-kHz stereo pilotsignal. PLL circuit 15 includes a phase comparator circuit 16, low passfilter 17, VCO 18, and first and second frequency dividers 19 and 20. Areference number 21 is a quadrature detector for effecting thequadrature detection of a compressed stereo difference signal S'included in an output signal from FM detection circuit 13 using a 38-kHz(90° phase angle) detection signal obtained from a third frequencydivider 23 of said PLL circuit 15. A reference number 22 is asynchronous detection circuit which detects an uncompressed stereodifference signal included in the output signal of FM detection circuit13 using a 38-kHz (0° phase angle) detection signal obtained from thethird frequency divider 23; and a reference number 24 is a variablephase shifter which shifts a 19-kHz stereo pilot signal and is locatedat the input side of PLL circuit 15.

Stereo sum signal M, uncompressed stereo difference signal S, compressedstereo difference signal S', and 19-kHz stereo pilot signal P areincluded in an output signal of FM detection circuit 13, and when asignal is passed through a low pass filter (not shown), stereo sumsignal M is obtained at a first output terminal 25.

Furthermore, the pilot signal P has its phase compared in a phasecomparator 16 with a 19-kHz signal obtained from a second frequencydivider 20 provided in PLL circuit 15, and a phase difference signalobtained therefrom is applied through a low pass filter 17 to VCO 18.Accordingly, VCO 18 produces an oscillation signal synchronized to thepilot signal P. The third frequency divider 23 divides the oscillationsignal from VCO 18, and produces two 38-kHz (0° and 90° phase angle)signals to be used as a detection signal. Quadrature detector 21 detectscompressed stereo difference signal S' included in the output of FMdetection circuit 13 using the aforementioned detection signal.Therefore, compressed stereo difference signal S' is produced at secondoutput terminal 26 which is connected to the output of quadraturedetector 21. Synchronous detector 22 synchronously detects uncompressedstereo difference signal S included in the output signal of FM detectioncircuit 13 using a 38-kHz (0° phase angle) signal obtained from thirdfrequency divider 23 as the detection signal. Therefore, uncompressedstereo difference signal S is produced from the third output terminal 27which is connected to the output of synchronous detector 22.

Compressed stereo difference signal S' and uncompressed stereodifference signal S obtained at second and third output terminals 26 and27 are applied to adder 3 shown in FIG. 3, FIG. 7, or FIG. 8, and afterexpansion processing, they are matrixed in matrix circuit 9 togetherwith stereo sum signal M produced from the first output terminal 25.

In the case where there is a difference in phase between the 38-kHzsignal obtained from PLL circuit 15 and the 38-kHz subcarrier, acrosstalk of stereo difference signal S occurs in quadrature detector 21and a crosstalk of compressed stereo difference signal S' occurs insynchronous detector 22. Accordingly, the linearity of the uncompressedstereo difference signal S deteriorates, resulting in such a problemthat, accurate expansion during expansion processing does not occur, andthat the linearity of output signal (S+S') of VCA 4 also deteriorates.Accordingly, this induces deterioration of stereo channel separationduring both FM stereo reception and FMX stereo reception.

In order to improve such a drawback, in the first embodiment show inFIG. 4, phase shift circuit 24 of a variable phase shift type isprovided between FM detector 13 and phase comparator 16 provided in PLLcircuit 15 to shift the phase to a correct position. The phase shiftcircuit 24 is composed of a phase delay circuit, such as shown in FIG.5, or a phase advance circuit, such as shown in FIG. 6. The phase shiftis carried out, for example, in such a manner that an FMX IF signalcontaining only the compressed stereo difference signal S' and not theuncompressed stereo difference signal S is applied to the input side ofFM detection circuit 13, and the phase is so shifted by phase shiftcircuit 24 so that the crosstalk of compressed stereo difference signalS' produced at the output of synchronous detector 22 will be made zero.Since the compressed stereo difference signal S' has a level 20 dBgreater than the uncompressed stereo difference signal S, a crosstalkwith respect to the output of synchronous detector 22 will be alsoextremely high. Also, since the output signal S of the synchronousdetector 22 is used as a reference signal during expansion, the abovementioned crosstalk especially has a significant influence ondeterioration of separation. Therefore, as discussed above, when thecrosstalk in the output of synchronous detector 22 is made zero byadjusting the phase shift amount of phase shift circuit 24, thereference level during expansion processing will be made accurate and,at the same time, the level of signal (S+S') applied to matrix circuit 9from VCA 4 of FIG. 3 will also be made accurate, thereby improving thestereophonic channel separation.

Insertion of phase shift circuit 24 at the input side of PLL circuit 15has been explained in the first embodiment shown in FIG. 4. As discussedabove, because it is sufficient if stereo difference signal S which isthe reference signal for expansion processing is correctly detected,insertion of phase shift circuit 24 at the input side of quadraturedetector 21 and synchronous detector 22, as illustrated in FIG. 9, isalso possible instead of insertion at the input side of PLL circuit 15.Moreover, the phase shift can be automatically corrected and theadjustment of the receiver can be simplified by generating a controlsignal through an arithmetic processing of the output signals ofsynchronous detector 22, quadrature detector 21, and adder 3 and, at thesame time, by constructing phase shift circuit 24 with a variablereactance circuit, so that phase shift circuit 24 automatically variesthe reactance of the variable reactance circuit according to the controlsignal.

As described above, according to the circuit shown in FIG. 4, because itis possible to match the phase of the detection signal produced from thePLL circuit and the phase of the detected signal produced from the FMdetection circuit, it is possible to correctly detect the uncompressedstereo difference signal and the compressed stereo difference signal.Therefore, deterioration of stereo channel separation can be prevented,and an FMX stereophonic broadcasting receiver with a superior sense ofsound can be provided.

Referring to FIG. 7, a circuit diagram of a receiver according to asecond embodiment of the present invention is shown. In the secondembodiment, the improvement is made in connection with the leveldetectors 5 and 6 shown in FIG. 3. In FIG. 7, a reference number 33 is afirst input terminal to which detected stereo sum signal M is applied;34 is a second input terminal to which detected uncompressed stereodifference signal S is applied; 35 is a third input terminal to whichdetected compressed stereo difference signal S' is applied; 36 is afirst low-pass filter which removes the high frequency component of thesignal applied to aforementioned first input terminal 33; 37 is a secondlow-pass filter connected to the input side of first level detectioncircuit 5; 38 is a third low-pass filter connected to the input side ofsecond level detection circuit 6; and 39 is a fourth low-pass filterwhich removes the high frequency component included in the signalobtained at the output side of switch 30. The description on thecircuits 3 through 8 which has been already described in connection withFIG. 1 will be omitted for the sake of brevity.

Because the high frequency component included in a signal applied tosecond input terminal 34 is removed by a second low-pass filter 37, onlythe uncompressed stereo difference signal S which does not include ahigh frequency component is applied to first level detection circuit 5.Therefore, an output signal correctly showing the level of theuncompressed stereo difference signal S is generated at the output sideof aforementioned first level detection circuit 5. Similarly, becausethe high frequency component included in an output signal of VCA 4 isremoved by third low-pass filter 38, the output signal of a second leveldetection circuit 6 correctly shows the level of signal (S+S'). Theoutput signals of first and second level detection circuits 5 and 6 arecompared in comparator 7, and a level difference signal produced fromcomparator 7 is applied to VCA 4. Accordingly, an output signal (S+S')from VCA 4 corresponds to stereo difference signal S. In this case, thesecond and third low-pass filters 37 and 38 are provided only to improvethe precision of level detection. Therefore, the signal which should bematrixed does not pass through the low pass filters and, thus, it is notnecessary to accurately match the phase characteristics of both low-passfilters.

A first low-pass filter 36 is provided to remove the high frequencycomponent included in stereo sum signal M, and a fourth low-pass filter39 is provided to remove the high frequency component included inuncompressed stereo difference signal S or output signal (S+S') of VCA 4which ever passes through switch 30. Since the output signals of thefirst and fourth low-pass filters 36 and 39 are to be applied to matrixcircuit 9 and matrixed, the phase characteristics of the first andfourth low-pass filters 36 and 39 must be accurately matched. In thiscase, adjusting the various characteristics of the two low-pass filtersis comparatively simple.

It is to be noted that first and fourth low-pass filters 36 and 39 canbe eliminated in a simple FMX stereophonic broadcasting receiver. Inthis case, a de-emphasis circuit connected to the output side of themultiplex circuit serves as a filter to remove the high frequencycomponent.

As described above, according to the circuit shown in FIG. 7, becausefilters are provided at the input side of each of first and second leveldetection circuits provided for expansion of the compressed stereodifference signal, it is not necessary to adjust the characteristics ofboth of the filters. Therefore, the circuit design and adjustmentprocedures can be simplified. Furthermore, because it is sufficient toprovided two filters even when a filter is inserted in the signal pathto be matrixed as shown in FIG. 7, adjustment of the variouscharacteristics of both filters can be done simply.

Referring to FIG. 8, a circuit diagram of a receiver according to athird embodiment of the present invention is shown. In the thirdembodiment, the improvement is made in connection with the leveldetectors 5 and 6 shown in FIG. 3. In FIG. 8, reference number 45 is afirst switch having an arm selectively connected to either a first fixedterminal which is connected to first input terminal 34 or a second fixedterminal which is connected to the output side of VCA 4; 46 is a leveldetection circuit the input side of which is connected to the movablearm of the first switch 45; 47 is a second switch having a movable armwhich is connected to the output side of aforementioned level detectioncircuit 46, and is selectively connected to either a first fixedterminal which is in turn connected to one input of comparator 7 or asecond fixed terminal which is in turn connected to the other input ofcomparator 7; 48 is a control circuit which controls switching of thefirst and second switches 45 and 47; and 49 and 50 are capacitors havinga low capacitance for maintaining the input signals of comparator 7.

When first and second switches 45 and 47 are switched to a position asshown in FIG. 8, according to the switch control signal generated fromcontrol circuit 48, stereo difference signal S applied to first inputterminal 34 passes first switch 45 and is applied to level detectioncircuit 46, and thus the level is detected. Furthermore, the outputsignal of level detection circuit 46 is transmitted to second switch 47and further to comparator 7, so that it is maintained by first capacitor49.

When first and second switches 45 and 47 are switched to a positionwhich is opposite to the position shown in FIG. 8, according to theswitch control signal generated from control circuit 48, output signal(S+S') of VCA 4 is transmitted through first switch 45 to leveldetection circuit 46, and thus the level is detected. Furthermore, theoutput signal of level detection circuit 46 is transmitted to secondswitch 47 and further to comparator 7, so that it is maintained bysecond capacitor 50. Accordingly, the voltages stored in the first andsecond capacitors 49 and 50 are compared in comparator 7 to produce thecontrol signal generated according to the difference therebetween. Thecontrol signal is then transmitted through rectifying circuit 8 to VCA4, and accordingly, level control in said VCA 4 is achieved. Therefore,a signal (S+S') greater than the knee-point level generated from adder 3is level controlled by VCA 4 to be expanded to approximately the samelevel as the uncompressed stereo difference signal S, and a signal(S+S') less than the knee-point level is level controlled by VCA 4 to beattenuated uniformly by approximately 20 dB.

It is to be note that the switch control signals for changing theswitching positions of the first and second switches 45 and 47 asgenerated from control circuit 48 has a frequency sufficiently high withrespect to the frequency of the uncompressed stereo difference signal Sand output signal (S+S') of VCA 4, the levels of the stereo differencesignal S and output signal (S+S') of VCA 4 can be correctly detected inlevel detection circuit 46.

As discussed above, according to the circuit shown in FIG. 8, the levelof stereo difference signal S and the level of VCA output signal (S+S')can be correctly detected by using a simple level detection circuit.Therefore, according to the present invention, an FMX stereophonicbroadcasting receiver of simple construction and having an expansioncircuit which operates without failure can be provided.

Although the present invention has been fully described with referenceto preferred embodiments, many modifications and variations thereof willnow be apparent to those skilled in the art, and the scope of thepresent invention is therefore to be limited not by the details of thepreferred embodiments described above, but only by the terms of theappended claims.

What is claimed is:
 1. In an FMX stereophonic receiver for receiving anFMX stereophonic broadcast signal which includes a stereo sum signal, anuncompressed stereo difference signal, and a compressed stereodifference signal which is modulated in quadrature with the uncompressedstereo difference signal and being compressed, wherein said FMXstereophonic receiver comprises:an FM detector for producing an FMdetection signal including a stereo pilot signal and a subcarriersignal, a PLL circuit for producing a first signal which is in asynchronized relationship with said stereo pilot signal included in saidFM detection signal and a second signal which is 90° phase shifted fromsaid first signal; a synchronous detection circuit for receiving said FMdetection signal and for producing said uncompressed stereo differencesignal in accordance with said first signal produced from said PLLcircuit; a quadrature detection circuit for receiving said FM detectionsignal and for producing said compressed stereo difference signal inaccordance with said second signal produced from said PLL circuit, and aphase shifting means for shifting the phase of at least one of said FMdetection signal applied to said PLL circuit and said FM detectionsignal applied to said detection circuits so as to correct the phasedifference between the stereo pilot signal contained in the FM detectionsignal being applied to said PLL circuit and the subcarrier signalcontained in the FM detection signal being applied to said synchronousdetection circuit and also to said quadrature detection circuit, therebysynchronizing the phases of said first signal and the subcarrier signalof the uncompressed stereo difference signal and also synchronizing thephases of said second signal and the subcarrier signal of the compressedstereo difference signal.
 2. An FMX stereophonic receiver as claimed inclaim 1 wherein said phase shifting means is inserted at the input sideof said PLL circuit.
 3. An FMX stereophonic receiver as claimed in claim1, further comprising:an adder for adding said uncompressed stereodifference signal and said compressed stereo difference signal; a levelcontrol circuit for controlling the level of the output signal obtainedfrom said adder; a first level detector for detecting the level of saiduncompressed stereo difference signal; a second level detector fordetecting the level of said signal produced from said adder; acomparator for comparing the output signals from said first and secondlevel detectors and for producing a difference signal representing adifference therebetween, said difference signal being used forcontrolling said level control circuit; a first filter for filtering thesignal applied to said first level detector; and a second filter forfiltering the signal applied to said second level detector.
 4. An FMXstereophonic receiver as claimed in claim 3, further comprising a thirdfilter connected in a line for transmitting said stereo sum signal and afourth filter connected in a line for transmitting said uncompressedstereo difference signal or the signal produced from said level controlcircuit.
 5. In an FMX stereophonic receiver for receiving an FMXstereophonic broadcast signal which includes a stereo sum signal, anuncompressed stereo difference signal, and a compressed stereodifference signal which is modulated in quadrature with the uncompressedstereo difference signal and being compressed, wherein said FMXstereophonic receiver comprises:an FM detector for producing an FMdetection signal including a stereo pilot signal; a PLL circuit forproducing a first signal which is in a synchronized relationship withsaid stereo pilot signal included in said FM detection signal; asynchronous detection circuit for receiving said FM detection signal andfor producing said uncompressed stereo difference signal in accordancewith said signal produced from said PLL circuit; a quadrature detectioncircuit for receiving said FM detection signal and for producing saidcompressed stereo difference signal in accordance with said signalproduced from said PLL circuit; a phase shifting means for shifting thephase of at least one of said FM detection signal applied to said PLLcircuit and said FM detection signal applied to said detection circuitsso as to correct the phase difference between the synchronizedrelationship signal and the signals to be detected in said detectioncircuits, said phase shifting means being inserted at the input side ofsaid detection circuits.
 6. In an FMX stereophonic receiver forreceiving an FMX stereophonic broadcast signal which includes a stereosum signal, an uncompressed stereo difference signal, and a compressedstereo difference signal which is modulated in quadrature with theuncompressed stereo difference signal and being compressed, wherein saidFMX stereophonic receiver comprises:an FM detector for producing an FMdetection signal including a stereo pilot signal; a PLL circuit forproducing a signal which is in a synchronized relationship with saidstereo pilot signal included in said FM detection signal; a synchronousdetection circuit for receiving said FM detection signal and forproducing said uncompressed stereo difference signal in accordance withsaid signal produced from said PLL circuit; a quadrature detectioncircuit for receiving said FM detection signal and for producing saidcompressed stereo difference signal in accordance with said signalproduced from said PLL circuit; a phase shifting means for shifting thephase of at least one of said FM detection signal applied to said PLLcircuit and said FM detection signal applied to said detection circuitsso as to correct the phase difference between the synchronizedrelationship signal and the signals to be detected in said detectioncircuits, an adder for adding said uncompressed stereo difference signaland said compressed stereo difference signal; a level control circuitfor controlling the level of the output signal obtained from said adder;a level detector for detecting the level of a signal applied thereto;first and second storing means; a first switching means for selectivelyconnecting said input of said level detector to a first connectedposition to receive said uncompressed stereo difference signal or to asecond connected position to receive the output signal from said levelcontrol circuit; a second switching means for selectively connectingsaid output of said level detector to a third connected position toapply the output of said level detector to said first storing means orto a fourth connected position to apply the output of said leveldetector to said second storing means; a switch control means forcontrolling said first and second switching means to make said first andthird connected positions at the same time, and to make said second andfourth connected positions at the same time, alternately at a frequencywhich is substantially higher than the frequency of said uncompressedstereo difference signal and of said signal produced from said levelcontrol circuit; and a comparator for comparing the signals stored insaid first and second storing means and for producing a differencesignal representing a difference therebetween, said difference signalbeing used for controlling said level control circuit.
 7. In an FMXstereophonic receiver for receiving an FMX stereophonic broadcast signalwhich includes a stereo sum signal, an uncompressed stereo differencesignal, and a compressed stereo difference signal which is formed bymodulating the uncompressed stereo difference signal by a quadraturemodulation and being compressed, wherein said FMX stereophonic receivercomprises:an FM detector for producing an FM detection signal includinga stereo pilot signal and a stereo sum signal; a PLL circuit for beinglocked with respect to said stereo pilot signal included in said FMdetection signal and for producing a first signal having a frequencyequal to that of a stereo subcarrier signal, and a second signal whichis 90° phase shifted signal of said first signal; a first synchronousdetection circuit for receiving said FM detection signal and forproducing said uncompressed stereo difference signal in accordance withsaid first signal produced from said PLL circuit; a second synchronousdetection circuit for receiving said FM detection signal and forproducing said compressed stereo difference signal in accordance withsaid second signal produced from said PLL circuit; and an adder ofadding said uncompressed stereo difference signal and said compressedstereo difference signal as produced from said first and secondsynchronous detection circuits; an expander for expanding said outputsignal from said adder in accordance with the signal produced from saidfirst synchronous detection circuit; selecting means for selectingeither one of the output signal from said first synchronous detectioncircuit or the output signal from said expander; a matrix circuit forproducing left and right stereo signals in accordance with said sumsignal included in said FM detection signal and the signal produced fromsaid selecting means; and a phase shifting means connected between saidFM detector and said first synchronous detection circuit for shiftingthe phase of said stereo subcarrier signal, thereby improving theseparation of the left and right stereo signals.
 8. In an FMXstereophonic receiver for receiving an FMX stereophonic broadcast signalwhich includes a stereo sum signal, an uncompressed stereo differencesignal, and a compressed stereo difference signal which is formed bymodulating the uncompressed stereo difference signal by a quadraturemodulation and being compressed, wherein said FMX stereophonic receivercomprises:an FM detector for producing an FM detection signal includinga stereo pilot signal and a stereo sum signal; a PLL circuit for beinglocked with respect to said stereo pilot signal included in said FMdetection signal and for producing a first signal having a frequencyequal to that of a stereo subcarrier signal, and a second signal whichis 90° phase shifted signal of said first signal; a first synchronousdetection circuit for receiving said FM detection signal and forproducing said uncompressed stereo difference signal in accordance withsaid first signal produced from said PLL circuit; a second synchronousdetection circuit for receiving said FM detection signal and forproducing said compressed stereo difference signal in accordance withsaid second signal produced from said PLL circuit; and an adder foradding said uncompressed stereo difference signal and said compressedstereo difference signal as produced from said first and secondsynchronous detection circuits; an expander for expanding said outputsignal from said adder in accordance with the signal produced from saidfirst synchronous detection circuit; selecting means for selectingeither one of the output signal from said first synchronous detectioncircuit or the output signal from said expander; a matrix circuit forproducing left and right stereo signals in accordance with said sumsignal included in said FM detection signal and the signal produced fromsaid selecting means; a level adjusting circuit for adjusting the levelof the output signal produced from said adder; a level detector fordetecting the level of a signal applied thereto; first and secondstoring means; a first switching means for selectively connecting saidinput of said level detector to a first connected position to receivesaid uncompressed stereo difference signal or to a second connectedposition to receive the output signal from said level control circuit; asecond switching means for selectively connecting said output of saidlevel detector to a third connected position to apply the output of saidlevel detector to said first storing means or to a fourth connectedposition to apply the output of said level detector to said secondstoring means; a switch control means for controlling said first andsecond switching means to make said first and third connected positionsat the same time, and to make said second and fourth connected positionsat the same time, alternately at a frequency which is substantiallyhigher than the frequency of said uncompressed stereo difference signaland of said signal produced from said level control circuit; and acomparator for comparing the signals stored in said first and secondstoring means and for producing a difference signal representing adifference therebetween, said difference signal being used forcontrolling said level control circuit.